This invention relates to a wavelenght/output power stabilizing apparatus of a semiconductor laser which is capable of stabilizing the wavelength/output power of a single-mode semiconductor laser (it is also referred to as a laser diode; LD).
In recent years, a semiconductor laser has been used for various equipment equipped with an optical system since the converting efficiency of an output energy with respect to an input energy is large. This semiconductor laser has such character as that its wavelength and output power are varied depending on variation of the operating temperature of the semiconductor laser. The wavelength/output power are also varied according to variation of injection current of a current supply source for supplying an injection current to the semiconductor laser (see a search report under the title of "Frequency and Power Stabilizations of GaAlAs Diode Laser on an Opto galvanic Effect" of Shingaku Giho Vol. 82 No. 218, OQE-99 of Technical Search Report of Denshitsunshin Gakkai; OQE82-95 to 106 (issued on Jan. 17, 1983)).
More specifically, the variation .DELTA..lambda. of the wavelength .lambda. of a semiconductor laser and the variation .DELTA.P of its output power P are expressed by a function of the variation of .DELTA.I of an injection current I and the variation .DELTA.T of the operating temperature T.sub.T of the semiconductor laser.
The relations therebetween are as follows; ##EQU1##
In the above relations, dT/dI represents the increase of temperature of the laser as a function of the injection current I injected by the semiconductor laser.
In the above research report, in order to stabilize the wavelength/output power, there are shown means for feeding back the variation of the wavelength output power to the operating temperature and feeding back the variation of the wavelength to the injection current and means for feeding back the variation of the output power to the injection current and feeding back the wavelength to the operating temperature. However, according to the afore-mentioned means, since the output power or wavelength is controlled by the feed-back to the operating temperature, with an environmental temperature variation, the system is slow in its responsive speed, and temporary variation of wavelength/output power is allowed.
If T equals constant, .DELTA.T=0, dT/dI=0, .differential.P/.differential.T=O are obtained. Therefore, the relations (A) and (B) can be rewritten as follows; ##EQU2##
These relations principly indicate that if the variation .DELTA.P of the output power P is constant, the variation .DELTA.I of the injection current I is restricted, and if the variation .DELTA.I of the injection current I is restricted, the variation .DELTA..lambda. of the wavelength is restricted, thus stabilizing the wavelength of the semiconductor.
Because of the above-mentioned phenomena, in order to stabilize the wavelength/output power of the semiconductor laser, it is preferred that the variation .DELTA.I of the injection current I is restricted while maintaining the operating temperature T.sub.T of the injection current I constant.
In view of the above, in order to maintain the operating temperature of the semiconductor laser at a specified temperature, it can be considered that a temperature controlling apparatus (see Japanese Patent Early Laid-open Publication No. 53-1782) having a Peltier element as a thermoelectric element is used. However, the employment of the temperature controlling apparatus has the following problems. That is, when a semiconductor laser is used, the semiconductor laser itself is heated by the injection current. Therefore, if the thermoelectric element is controlled in such a way as to bring the operating temperature close to the specified temperature based on the temperature difference between the operating temperature of the semiconductor laser and the specified temperature, the operating temperature differs from the specified temperature due to variations of the environmental temperature caused by the heating of the semiconductor laser. In addition, the operating temperature is difficult to be stabilized for a long time due to seccessive variation of the thermal resistance of the semiconductor laser element and/or successive variation of the element forming the temperature controlling apparatus.
The above-mentioned problems will be described more in detail with reference to FIG. 1 and 2.
FIG. 1 is a schematic view of an operating temperature controlling portion for stabilizing the operating temperature of a semiconductor laser 1. FIG. 2 is a schematic view of a thermoelectric converting apparatus 6 for stabilizing the operating temperature of the semiconductor laser 1. The thermoelectric converting apparatus 6 is provided with the semiconductor laser 1 on one side of a Peltier element 7 and with a radiating plate 8 on the other side thereof. The thermoelectric converting apparatus 6 contains a thermistor 9.
The thermistor 9 detects the operating temperature T.sub.T of the semiconductor laser 1, and the operating temperature T.sub.T is voltage-converted into an operating temperature converting voltage E.sub.T by a temperature voltage converting circuit 32. This operating temperature converting voltage E.sub.T is inputted into one terminal of an operational amplifier 10. The other terminal of the operational amplifier 10 is inputted with a reference voltage E.sub.S corresponding to a specified temperature T.sub.S by a reference power source 11. The operational amplifier 10 compares the reference voltage E.sub.S and the operating temperature converting voltage E.sub.T and outputs the different output towards a transistor 12. The transistor 12 comprises a transistor 12a and a transistor 12b. The current flowing direction of the Peltier element 7 is converted by the transistor 12 so that when E.sub.T &gt;E.sub.S and T.sub.T &gt;T.sub.S, the transistor 12 is controlled so that the semiconductor laser 1 is cooled by the Peltier element 7, whereas when E.sub.T &lt;E.sub.S (T.sub.T &lt;T.sub.S), the transistor 12 is controlled so that the semiconductor laser 1 is heated by the Peltier element 7. As a result, the operating temperature T.sub.T of the semiconductor laser 1 is controlled as as to approach to the specified temperature T.sub.S, reaches a balance-state and becomes a balanced-temperature Te.
In the operating temperature controlling portion, however, the operating temperature T.sub.T is varied based on variation of the environmental temperature T.sub.h and heat value of the semiconductor laser 1.
For example, the Peltier element 7 has such characteristics as shown in FIG. 3. The illustration of FIG. 3 showing the characteristics is based on the Peltier element 7 of KSM-0211 manufactured by Komatsu Electronics. In this FIG. 3, the vertical axis shows a heat value Q as a heating load incurred to the Peltier element 7, whereas the horizontal axis shows a balanced-current I.sub.P which flows in the Peltier element 7. The parameter T is a temperature difference between the operating temperature T.sub.T when it reaches the balanced-state (T.sub.T =T.sub.T at this time) and the environmental temperature T.sub.h as a temperature at the radiating side of the Peltier element 7 and is represented by the following relation; EQU .DELTA.T.ident.Te-T.sub.h
The temperature difference T=0 means that the balanced-temperature Te is equal to the environmental temperature T.sub.h.
As is apparent from FIG. 3, when a heating element (Q.noteq.0) is used, even when, for example, the temperature difference T=0.degree. C., a balanced-current I.sub.p flows in the Peltier element 7 so as to release a heat value Q. In this case, the operating temperature converting voltage E.sub.T when the operating temperature T.sub.T reaches the balanced-temperature Te is a balanced-temperature corresponding voltage Ee. Furthermore, if the voltage/current converting coefficient of the operating temperature contolling portion shown in FIG. 1 is represented by .alpha., the balanced-temperature corresponding voltage Ee corresponding to the balanced-temperature of the semiconductor laser 1 when the heat is balanced can be obtained by the following relation; ##EQU3## which is obtained by rewriting the relation of I.sub.1.sup.e =(Ee-E.sub.S).
In the above relation, I.sub.1 represents a current flowing in the Peltier element 7 when the specified temperature T.sub.S and the environmental temperature T.sub.h are equally controlled. At that time, a relation of E.sub.S =E.sub.h is obtained between the reference voltage E.sub.S and the environmental temperature corresponding voltage E.sub.h.
Furthermore, since the balanced current I.sub.p.sup.e and the heat value Q are in linear relation when the heat value Q is in the small range as shown in FIG. 3, the heat value Q can be represented by the following relation; EQU Q=.beta..multidot.I.sub.1.sup.e ( 2)
wherein .beta. is a converting coefficient.
Therefore, based on the relations (1) and (2), the balanced-temperature corresponding voltage Ee can be obtained by the following relation; ##EQU4##
This relation (3) shows that it becomes Ee=E.sub.S (.DELTA.T=0) since the controlling circuit controls so as to make E.sub.S -E.sub.T =E.sub.S -Ee=0 when the reference voltage E.sub.S is set to be equal to the environmental temperature corresponding voltage E.sub.h when Q=0. It also shows that it becomes as follow; EQU Ee.noteq.E.sub.S ( 4)
even if it is set as E.sub.S =E.sub.h so as to set the specifiedting temperature T.sub.S and environmental temperature T.sub.h equally when Q.noteq.0. That is, this relation shows that when a heating element such as the semiconductor laser 1 is used, the balanced-temperature corresponding voltage Ee corresponding to the balanced temperature Te is not equal to the reference voltage E.sub.S corresponding to the specified temperature T.sub.S and that in this operating temperature stabilizing circuit, the balanced-temperature Te is shifted with respect to the specified temperature T.sub.S by a value which is in proportion to the size of the heat value Q, i.e., a value which is equal to Q/(.alpha..multidot..beta.). The heat value Q is in proportion to the injection current I of the semiconductor laser 1.
The environmental temperature T.sub.h is not constant but variable. The specified temperature T.sub.S is not necessarily in agreement with the environmental temperature T.sub.h. When the balanced-temperature Te is different from the environmental temperature T.sub.h (.DELTA.T=Te-T.sub.h .noteq.0), a balanced-current I.sub.2 is flowed in the Peltier element 7 even when the heating element is not used as shown in FIG. 3. FIG. 4 is an illustration of the characteristics of the Peltier element 7 in which the relation between .DELTA.T=Te-T.sub.h and the balanced-current I.sub.2.sup.e when Q=0. The balanced temperature corresponding voltage Ee can be obtained from the following relation; ##EQU5## which is obtained by rewriting I.sub.2.sup.e =.alpha.(Ee-E.sub.S). In the portion where the temperature T between the balanced temperature Te and the environmental temperature T.sub.h is small (.DELTA.T.ltoreq.15.degree. C.), the temperature difference .DELTA.T and the balanced-current I.sub.2.sup.e are in linear relation. Therefore, the temperature difference .DELTA.T can be represented by the following relation; EQU .DELTA.T=-.gamma..multidot.I.sub.2.sup.e ( 6)
wherein the flowing direction of the balanced-current I.sub.2.sup.e flowing in the Peltier element 7 is normal when it flows in the direction for cooling the semiconductor laser 1 as a test sample, and .gamma. is a converting coefficient.
By using this relation (6), if the balanced-temperature corresponding voltage Ee is shown as a function of the temperature difference .DELTA.T by rewriting the relation (5), it can be represented by the following relation; ##EQU6##
Accordingly, if the operating temperature controlling portion which is shown in FIG. 1 is used, the balanced-temperature corresponding voltage Ee corresponding to the balanced-temperature Te is not in accord with the reference voltage E.sub.S when the specified temperature T.sub.S and the environmental temperature T.sub.h are not in accord. The differnece Ee-E.sub.S means that the balanced-temperature Te is shifted by a value which is in proportion to .DELTA.T with respect to the specified temperature T.sub.S.
More specifically, even if the specified temperature T.sub.S is constant, the temperature difference T varies when the environmental temperature T.sub.h varies. Accordingly, since the balanced-temperature Te is varied by the influence of the environmental temperature T.sub.h, the operating temperature T.sub.T is not constant.
Next, when a heating element is used, and the environmental temperature T.sub.h and the specified temperature T.sub.S are not in agreement with respect to each other, the balanced-current I.sub.p is represented according to the principle of superimposition by the following relation; ##EQU7##
This relation (8) can be rewritten according to the relation (1) as follows; ##EQU8## and the following relation can then be obtained; ##EQU9## In this way, the balanced-temperature Te (corresponding voltage Ee) varies according to the heat value Q of the semiconductor laser and variation of the difference T between the specified temperature T.sub.S and the environmental temperature T.sub.h.
Accordingly, it is unexpected that the operating temperature T.sub.T is stabilized for a long time by such operating temperature controlling portion as mentioned. Furthermore, even if it is assumed that the operating temperature T.sub.T at the place where the thermistor 9 is disposed is constant, there is no guarantee that the operating temperature T.sub.T of the semiconductor laser 1 is stabilized for a long time due to successive variation of thermal resistance between the thermistor 9 and the semiconductor laser 1 and successive variation of the thermistor 9 itself. There is no guarantee either that the output power P itself is constant since the operating temperature T.sub.T is not controlled by considering the variation of the output power P.
Therefore, it is difficult to stabilize both the wavelength of the semiconductor laser 1 and output power thereof.